This invention relates in general to switched reluctance drives and more particularly to a commutator for controlling the firing of phases of a switched reluctance motor over a very wide speed range.
Although they have been known for some time, interest in switched reluctance motor (SRM) drives has recently revived. Compared to conventional induction and synchronous motor drive systems, the SRM drive is simple in construction and economical. In addition, the converter which supplies power to the SRM machine requires fewer power devices and therefore is more economical and reliable. In view of these advantages, the switched reluctance motor drive system provides an attractive alternative to conventional drive systems and is expected to find wide applicability in industrial applications.
Switched reluctance motors conventionally have multiple poles or teeth on both the stator and rotor (i.e., double salient). There are phase windings on the stator but no windings or magnets on the rotor. Each pair of diametrically opposite stator poles is connected in series to form an independent phase of the multiphase switched reluctance motor.
Torque is produced by switching current on in each phase winding in a predetermined sequence that is synchronized with the angular position of the rotor, so that a magnetic force of attraction results between the rotor and stator poles that are approaching each other. The current is switched off in each phase before the rotor poles nearest the stator poles of that phase rotate past the aligned position; otherwise, the magnetic force of attraction would produce a negative or braking torque. The torque developed is independent of the direction of current flow so that unidirectional current pulses synchronized with rotor movement can be applied to the stator phase windings by a converter using unidirectional current switching elements such as thyristors and transistors.
The switched reluctance drive thus operates by switching the stator phase currents on and off in synchronism with rotor position. By properly positioning the firing pulses relative to rotor angle, forward or reverse operation and motoring or generating operation can be obtained.
In many cases, use of a fixed set of current pulse firing angles in conjunction with current level regulation suffices to control torque for the entire range of motor operation. In such cases, a set of optical interrupters and a slotted disk can be used to perform commutation.
Recently, however, the switched reluctance motor has been finding increasing application as a combination motor/generator, or as a position servo. For these applications, fixed firing angles do not always provide sufficient machine torque performance over the required speed range. Also for these applications, precise position and speed information is often required over the full operating range of the machine. In such applications, a slotted disk may not provide sufficient accuracy and an optical encoder or resolver is often used for position and speed sensing.
In general, as the speed range of a switched reluctance motor increases, it becomes desirable to be able to selectively adjust the firing angles of the current pulses with respect to rotor position. At high speeds, current control is lost and the only way to control the motor torque is by varing pulse position and width.
If an SRM analog controller is used in such applications, commutator circuitry in hardware form is needed to generate the appropriate phase firing pulses. When, as is often desirable, a microprocessor-based controller is employed, the processor itself can generate the firing commands at lower speeds, but as the machine's speed increases, this task takes an unacceptable portion of the processor's time. Accordingly, it is desirable to shift the commutation function from the microprocessor to a separate digital circuit.
A number of digital commutator circuits have previously been proposed for brushless DC motors. Most of these appear to be limited in either their interface to the rotor position sensor or their speed range. By way of example, L. Thompson and M. Lee in a paper intitled, "Universal Brushless Motor Commutator" presented at the 13th Annual Symposium on Incremental Motion Control Systems and Devices, Urbana, Ill. May, 1984, describe a commutation circuit that interfaces to an incremental encoder and allows firing advance, but uses a fixed pulsewidth. However, in a switched reluctance motor, current regulation is unavailable at high speed because of back EMF build-up, and pulsewidth widening is needed to allow operation at such high speeds.
U.S. Pat. Nos. 4,270,074 and 4,368,411 describe control systems for brushless DC motors employing a read-only memory (ROM). By brushless DC motors, these patents, as is conventional, refer to permanent magnets are glued to the surface of the rotor and the stator phase windings are connected together in a WYE configuration, rather than to switched reluctance motors. U.S. Pat. No. 4,270,074 employs a ROM addressed by motor shaft position sensors to ensure synchronous operation, but makes no provision for changing the response of the memory as a function of speed. U.S. Pat. No. 4,368,411 uses a ROM to control a drive switching circuit. An external pulsewidth modulator allows some variation in pulsewidth of the pulses produced by the ROM but this patented circuit does not provide variation in the turn-on angle of these pulses.
Accordingly, need exists for a commutator to control the switching of stator phases of a switched reluctance motor in a digital control scheme, relieving the microprocessor of that task. This would also allow great flexibility in both the placement and duration of phase firing pulses, thereby making SRM performance possible over a wider speed range including extremely high speeds.